This invention relates generally to semiconductor wafer polishing, and in particular to an apparatus and process for polishing peripheral edges of wafers at elevated temperature for improved throughput.
Semiconductor wafers are generally prepared from a single crystal ingot, such as a silicon ingot, which is sliced into individual wafers. Each wafer is subjected to a number of processing operations to facilitate the installation of integrated circuit devices and to improve their yield, performance, and reliability. Typically, these operations reduce the thickness of the wafer and remove damage caused by the slicing operation. Chemical-mechanical polishing of semiconductor wafers is one of these operations. It generally involves rubbing a wafer with a polishing pad, such as a polyurethane impregnated polyester felt, while dispensing a polishing solution, or slurry. The slurry contains an abrasive and chemicals, such as a colloidal silica and an alkaline etchant, so that both mechanical action and chemical reaction contribute to the removal of material. The polishing process produces surfaces that are flat, highly reflective, and damage-free.
It is crucial that each wafer undergo polishing not only on at least one of its flat, facial surfaces but also along its peripheral edge. Wafer edges frequently contain micro-cracks and chip damage from the slicing operation and from a grinding operation, or profiling, along the edges that forms a selected edge profile shape. During handling and processing, edges often receive impact forces or high local stresses that cause additional fractures, chips, or roughness. If left in place, these imperfections become local stress points for the nucleation of damage to the lattice of the crystal structure. They significantly increase the likelihood of subsequent growth of slips and dislocations that can ruin the wafer. Further, edge roughness tends to facilitate adherence of impurities, such as dust particles and inorganic anions. These impurities can diffuse from wafer edges to facial surfaces and detrimentally contaminate wafers. Accordingly, edges are polished to remove imperfections or reduce their size.
One type of machine that is used for polishing semiconductor wafer edges is shown in U.S. Pat. No. 5,094,037, which is hereby incorporated by reference. The machine includes at least one rotatable wafer holder that presses the edge of the wafer against a rotating cylindric drum. The drum is covered with a polishing pad so that as the drum rotates, the polishing pad rubs against the edge of the wafer. The drum oscillates vertically as it rotates, evenly exposing all portions of the polishing pad against the wafer edge. Simultaneously, the wafer holder rotates the wafer in a direction opposite to that of the drum, thereby increasing the relative speed between the wafer edge and the polishing pad. Slurry may be prepared in a container having an agitator for mixing the slurry and maintaining a preselected pH level. The slurry is dispensed at a controllable flowrate onto the polishing pad at a location where the wafer engages the pad.
A limitation to the chemical mechanical polishing process, including machines of this type, is that polishing requires a substantial amount of time. A typical duration for the chemical reaction and mechanical action to effect an acceptable level of smoothness along a first side or bevel of an edge is 85 seconds. That time duration must then be repeated to polish a second, opposite side of the edge. To shorten the time, polishing has been conducted at elevated temperatures which accelerates the chemical reaction between the slurry and the wafer. However, there have been difficulties implementing high temperature edge polishing and there are no machines of the type described above that permit it. Throughput with edge polishing machines is typically limited to between 32 and 42 wafers per hour, and the edge polishing process is a hindrance to efficient processing of wafers.